Plasma sheath in a tilted magnetic field: Closing of the diamagnetic currents; effect on plasma convection

A plasma on open field lines that intersect endplates at a shallow angle, as in a tokamak with a toroidal limiter or poloidal divertor, is considered. A microscopic picture is developed, which shows the closure of the electron diamagnetic and electric drift currents approaching the wall by parallel (to B) electron current. A proper account of the radial variation of the ion ‘‘skin’’ current in the ion gyrosheath is made. The resulting sheath current-voltage characteristics are derived. Two applications are considered: the toroidal potential variations induced by a wavy limiter or divertor surface, which can lead to significant plasma convection, and the determination of the boundary conditions appropriate for macroscopic instabilities in such a configuration.